1. Field of the Invention
This invention relates to a fine motion stage for high precision alignment of movable reticle and wafer stages in a scanning exposure apparatus for semiconductor lithography, as well as to other precision motion apparatuses.
2. Description of the Prior Art
In semiconductor lithography as used to fabricate integrated circuits, conventionally a reticle (mask) is imaged through a reduction-projection lens onto a semiconductor wafer located below the lens. A scanning exposure device uses simultaneous motion of the reticle and wafer (each supported on its own movable stage) to continuously project an image of a portion of the reticle onto the wafer through the projection lens and thereby expose a photosensitive resist layer on the wafer. Scanning, as opposed to exposure of the entire reticle at once, allows for projection of reticle patterns that exceed in size that of the image field of the projection lens.
Present FIG. 1 is similar to FIG. 1 of Nishi U.S. Pat. No. 5,477,304 issued Dec. 19, 1995 entitled "Projection Exposure Apparatus" assigned to Nikon Corporation and incorporated herein by reference. This illustrates a slit scanning exposure apparatus for semiconductor processing. A reticle base 19 is part of the stage system for a reticle 7. A reticle scanning stage 20 is placed on the reticle base 19 to be slideable in the X direction within the XY plane where the X axis is perpendicular to the drawing surface of FIG. 1 and the Y axis is parallel to the drawing surface of FIG. 1. The reticle 7 is held on the reticle fine adjustment stage 21. In an exposure operation a pattern area of the reticle 7 is illuminated by exposure light IL from an illumination optical system 22 in the form of a rectangular illumination area (i.e. a slit), and the reticle 7 is scanned in the X direction with respect to the slit-like illumination area. The illumination optical system 22 is conventional.
Three movable mirrors (only a single movable mirror 33 is shown) are disposed on the reticle fine adjustment stage 21. Three laser interferometers (only a single laser interferometer 35 is shown) obtain the positions and rotational angles of the reticle fine adjustment stage 21 within the XY plane. The measurement results obtained by these laser interferometers (position detectors) are supplied to a main control system 23. The main control system 23 controls the operation of the reticle scanning stage 20 through a relative scanning drive 24, and also controls operation of the reticle fine adjustment stage 21 through a fine adjustment drive 25.
In the exposure operation, a pattern in the slit-like illumination area under reticle 7 is projected/exposed on the wafer 14 through a projection optical system 13.
In the stage system for the wafer 14, an air guide elongated in the X direction is found on the wafer base 26 and a wafer X stage 27 is placed on the wafer base 26 to be slideable in the X direction in the XY plane. A wafer Y stage 28 is placed on the wafer X stage 27 to be movable in the Y direction within the XY plane. The wafer 14 is held on the wafer Y stage 28. Also present are a Z stage, a levelling stage and other elements conventionally arranged between the wafer Y stage 28 and the wafer 14. A stepping motor 29 is disposed on one end of the wafer X stage 27. The stepping motor 29 drives the wafer Y stage 28 in the Y direction by a ball screw 30.
Three movable mirrors (only a single movable mirror 45 is shown) are disposed on the wafer Y stage 28. Three laser interferometers (only a single laser interferometer 47B is shown) obtain the positions and rotational angles of the wafer Y stage 28 within the XY plane conventionally. The measurement results obtained by these laser interferometers are also supplied to the main control system 23. In accordance with the three position measurement results, the main control system 23 controls the operation of the wafer X stage 27 and the wafer Y stage 28 through a drive 31.
A typical control system of the type used in FIG. 1 for the reticle and wafer stages is shown schematically in FIG. 2 (FIGS. 2-4 use different reference numbers than does FIG. 1 for equivalent structures for greater clarity). High precision position synchronization of the (coarse) actuator for reticle stage 110 and the (coarse) actuator for wafer stage 112 is effected by a high-bandwidth, fine motion stage 116 driven by a PZT-type actuator 117 mounted on the reticle stage 110. (The reticle and wafer stages are shown for purposes of simplicity combined with their respective actuators.) Controller 120 provides control signals to fine motion stage amplifier 118, reticle stage actuator servo amplifier 122, and wafer stage actuator servo amplifier 124. Conventional position sensors (as described above and not shown here) sense the position of each of the fine stage 116, the reticle stage actuator 110, and wafer stage 112 and provide position indication signals to controller 120 which is e.g. a programmed microprocessor or microcontroller.
A motion control system for all three stages of FIG. 2 is shown schematically in FIG. 3. The input and output signals of FIG. 3 represent functions of the positions of the stages. The reticle stage 110 trajectory as illustrated is a constant multiple (for example, four) of the position command 115 from controller 120 and hence is four times that of the wafer stage 112. Also, the motions of reticle stage 110 and wafer stage 112 are actually in opposite directions because the projection lens of the type in the system of FIG. 1 typically is a reversing-type optics. These two factors account for the multiplications by -4 and 4, respectively, shown at the reticle stage 110 input (drive) signal and the wafer stage 112 output (position indication) signal in FIG. 3.
A typical closed-loop fine motion stage control feedback system for the fine motion stage 116 and its actuator 117 of FIG. 3 is shown in detail in block diagram form in FIG. 4. The input signal ("Desired Position") is the desired trajectory correction corresponding to the positional deviation between the reticle stage 110 and wafer stage 112, and the output signal ("Actual Position") is the actual position achieved by the fine motion stage 116. In addition, there is often a noise disturbance independent of the controller 120. Thus the output signal is the sum of a controlled output signal from the fine motion stage 16 and its actuator 117 plus a noise contribution, which together result in a position signal determined by the conventional position sensor associated with the fine motion stage 116 which senses e.g. the location of a side of fine motion stage 116.
The controller 120 functionally is characterized mathematically as a Laplace transform having a gain k, two real zeroes a1, a2, a simple pole at the origin, and a real simple pole b2. Mathematically, this function may be expressed as EQU G(s)=k*(s+a1) (s+a2)/(s*(s+b2)),
where k, a1, a2 and b2 are constants chosen to achieve the desired controller performance and s is the Laplace transform complex operator. Typically b2&gt;a2 and the value of b2 depends on the system structural frequency. Typically a2&gt;a1 and the value of a2 depends on the system stability.
However, this prior art controller has the disadvantage that its performance (as measured by error convergence rate, noise reduction and stability), is determined solely by the particular constants that characterize the controller transfer function. Thus the characteristics of the controller cannot be varied with respect to the continuously changing instantaneous error.